1. Field of the Invention
The present invention relates to a communications receiver system, and more specifically to the acquisition of a received quadrature amplitude modulated (QAM) signal containing a frequency offset.
2. Related Art
Communication systems require convergence of several signal processing algorithms before a communications receiver can output meaningful data. One such signal processing algorithm uses a timing recovery process to obtain symbol synchronization. Symbol synchronization involves determining a sample frequency and a sample phase of a received symbol. The determination of the sample frequency requires an estimate of a period of the symbol to sample a received communication signal at a correct sampling rate. The determination of the sample phase involves determining the correct time within a symbol period to take a sample.
Commonly, after obtaining symbol synchronization, another signal processing algorithm uses a carrier recovery process to remove unknown frequency offsets from the received communication signal. Ideally, the frequency of an oscillator in the communication receiver will match the frequency of an oscillator used at a communication transmitter. In practice, their frequencies differ. For example, any variation in the oscillator of the communication receiver can cause a frequency difference between these oscillators. When the frequency of the transmitter oscillator differs from the frequency of the receiver oscillator, the process of down-conversion results in an unknown offset in the frequency content of the received communication signal relative to the transmitted communication signal. The receiver may use a carrier recovery loop to remove these unknown frequency offsets from the symbol content of the received communication signal.
However, a conventional carrier recovery process may take a prohibitively long duration to converge, or may simply not converge at all, in the presence of a large unknown frequency offset. For example, a ±300 kHz carrier frequency offset represents a large unknown frequency offset for a quadrature amplitude modulation (QAM) signal in a cable modem. The conventional carrier recovery process requires approximately 100 ms to converge in the presence of this ±300 kHz carrier frequency offset. In a conventional receiver in some applications (such as downstream cable, e.g., DOCSIS) many channels may be required to attempt frequency locking and data acquisition, before finding a channel containing a valid QAM signal, Thus, when a long frequency acquisition time per channel must be accommodated, when multiplied by many, such as hundreds to provide an example, of possible communications channels containing a QAM signal, the convergence time becomes a prohibitively long duration.
What is needed is a carrier recovery process that converges in the presence of the large unknown frequency offset that overcomes the shortcomings descried above. Further aspects and advantages of the present invention will become apparent from the detailed description that follows.
The present embodiments will now be described with reference to the accompanying drawings. In the drawings, like reference numbers may indicate identical or functionally similar elements.